95 lines
3.8 KiB
Markdown
95 lines
3.8 KiB
Markdown
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# proc-macro2
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[<img alt="github" src="https://img.shields.io/badge/github-dtolnay/proc--macro2-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/proc-macro2)
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[<img alt="crates.io" src="https://img.shields.io/crates/v/proc-macro2.svg?style=for-the-badge&color=fc8d62&logo=rust" height="20">](https://crates.io/crates/proc-macro2)
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[<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-proc--macro2-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" height="20">](https://docs.rs/proc-macro2)
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[<img alt="build status" src="https://img.shields.io/github/workflow/status/dtolnay/proc-macro2/CI/master?style=for-the-badge" height="20">](https://github.com/dtolnay/proc-macro2/actions?query=branch%3Amaster)
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A wrapper around the procedural macro API of the compiler's `proc_macro` crate.
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This library serves two purposes:
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- **Bring proc-macro-like functionality to other contexts like build.rs and
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main.rs.** Types from `proc_macro` are entirely specific to procedural macros
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and cannot ever exist in code outside of a procedural macro. Meanwhile
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`proc_macro2` types may exist anywhere including non-macro code. By developing
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foundational libraries like [syn] and [quote] against `proc_macro2` rather
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than `proc_macro`, the procedural macro ecosystem becomes easily applicable to
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many other use cases and we avoid reimplementing non-macro equivalents of
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those libraries.
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- **Make procedural macros unit testable.** As a consequence of being specific
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to procedural macros, nothing that uses `proc_macro` can be executed from a
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unit test. In order for helper libraries or components of a macro to be
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testable in isolation, they must be implemented using `proc_macro2`.
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[syn]: https://github.com/dtolnay/syn
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[quote]: https://github.com/dtolnay/quote
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## Usage
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```toml
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[dependencies]
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proc-macro2 = "1.0"
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```
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The skeleton of a typical procedural macro typically looks like this:
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```rust
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extern crate proc_macro;
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#[proc_macro_derive(MyDerive)]
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pub fn my_derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
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let input = proc_macro2::TokenStream::from(input);
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let output: proc_macro2::TokenStream = {
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/* transform input */
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};
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proc_macro::TokenStream::from(output)
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}
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```
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If parsing with [Syn], you'll use [`parse_macro_input!`] instead to propagate
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parse errors correctly back to the compiler when parsing fails.
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[`parse_macro_input!`]: https://docs.rs/syn/1.0/syn/macro.parse_macro_input.html
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## Unstable features
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The default feature set of proc-macro2 tracks the most recent stable compiler
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API. Functionality in `proc_macro` that is not yet stable is not exposed by
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proc-macro2 by default.
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To opt into the additional APIs available in the most recent nightly compiler,
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the `procmacro2_semver_exempt` config flag must be passed to rustc. We will
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polyfill those nightly-only APIs back to Rust 1.31.0. As these are unstable APIs
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that track the nightly compiler, minor versions of proc-macro2 may make breaking
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changes to them at any time.
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```
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RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo build
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```
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Note that this must not only be done for your crate, but for any crate that
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depends on your crate. This infectious nature is intentional, as it serves as a
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reminder that you are outside of the normal semver guarantees.
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Semver exempt methods are marked as such in the proc-macro2 documentation.
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<br>
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#### License
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<sup>
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Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
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2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
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</sup>
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<br>
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<sub>
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Unless you explicitly state otherwise, any contribution intentionally submitted
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for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
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be dual licensed as above, without any additional terms or conditions.
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</sub>
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